Abstract
BackgroundPlasticity in brain size and the size of different brain regions during early ontogeny is known from many vertebrate taxa, but less is known about plasticity in the brains of adults. In contrast to mammals and birds, most parts of a fish’s brain continue to undergo neurogenesis throughout adulthood, making lifelong plasticity in brain size possible. We tested whether maturing adult three-spined sticklebacks (Gasterosteus aculeatus) reared in a stimulus-poor environment exhibited brain plasticity in response to environmental enrichment, and whether these responses were sex-specific, thus altering the degree of sexual size dimorphism in the brain.ResultsRelative sizes of total brain and bulbus olfactorius showed sex-specific responses to treatment: males developed larger brains but smaller bulbi olfactorii than females in the enriched treatment. Hence, the degree of sexual size dimorphism (SSD) in relative brain size and the relative size of the bulbus olfactorius was found to be environment-dependent. Furthermore, the enriched treatment induced development of smaller tecta optica in both sexes.ConclusionsThese results demonstrate that adult fish can alter the size of their brain (or brain regions) in response to environmental stimuli, and these responses can be sex-specific. Hence, the degree of SSD in brain size can be environment-dependent, and our results hint at the possibility of a large plastic component to SSD in stickleback brains. Apart from contributing to our understanding of the processes shaping and explaining variation in brain size and the size of different brain regions in the wild, the results show that provision of structural complexity in captive environments can influence brain development. Assuming that the observed plasticity influences fish behaviour, these findings may also have relevance for fish stocking, both for economical and conservational purposes.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-015-0130-0) contains supplementary material, which is available to authorized users.
Highlights
Plasticity in brain size and the size of different brain regions during early ontogeny is known from many vertebrate taxa, but less is known about plasticity in the brains of adults
The treatment had a sex-specific effect on total brain volume (Table 1): males had generally larger brains, and only males responded to enrichment by increasing their relative brain size (Fig. 1a)
The male-biased sexual size dimorphism (SSD) in the relative size of the total brain was higher in the enriched (SSD = 4.1 %) than in the simple treatment (SSD = 2.5 %; Fig. 1a). Another sex × treatment interaction was observed (Table 1): males in the enriched treatment decreased the relative size of their bulbus olfactorius compared to the other groups (Fig. 1b)
Summary
Plasticity in brain size and the size of different brain regions during early ontogeny is known from many vertebrate taxa, but less is known about plasticity in the brains of adults. In contrast to higher vertebrates with determinate growth, localized and limited adult neurogenesis [20,21,22] lower vertebrates with indeterminate growth are characterised by neurogenesis that generally persists longer into adulthood. This facilitates lifelong brain growth, and potentially plasticity in brain size and the size of different brain regions in response to spatial and temporal environmental variability, even in adults [7, 23,24,25,26]. Park et al [27] reported that adult three-spined sticklebacks (Gasterosteus aculeatus) collected from the wild appeared to experience a reduction in relative
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